This invention relates generally to light-load actuators for converting rotary motion to linear motion, as in boat steering mechanisms and the like and more particularly to devices of this type which utilize ball elements to reduce the friction and improve the overall drive efficiency.
In the past a number of different types of small, ball-type traverses or transmissions have been proposed and produced, meeting with varying degrees of success. A somewhat similar but heavy-duty prior arrangement is disclosed in U.S. Pat. No. 3,296,880, issued to Arthur M. Maroth on Jan. 10, 1967. The device described therein consists of a combination of screw and nut wherein the latter is provided with multiple roller elements, each having a tapered or conical body which is cooperable with the wall of the screw thread. The rollers are carried on suitable needle bearings, and as the nut rotates with respect to the screw, the rollers turn, thereby greatly increasing the efficiency of the transmission by substantially completely eliminating all sliding friction between the conical roller bodies and the screw threads. Under such circumstances, the friction between the rollers and the screw thread is of the rolling kind.
While the above device functioned in an acceptable manner, its efficient operation relied to a great extent on maintaining the proper dimensions and tolerances of both the roller elements and the thread formation. Such tolerances required precise machining in many cases, resulting in a device which generally worked well but was relatively expensive to produce. Moreover, due to the existence of the precision parts, the assembly of the device was somewhat complex and time-consuming. Also, care had to be exercised in order to insure that the multiple moving parts associated with the rollers, i.e. the components making up the bearings therefor, were adequately lubricated.
Yet another type of transmission, of lighter-duty rating, is disclosed in U.S. Pat. No. 4,031,765 issued to Joseph R. Metz on June 28, 1977. The device illustrated therein involves a reciprocating nut which is carried by a screw having a diamond-thread configuration. Multiple balls are carried by the nut and extend into the bore thereof, for engagement with the walls of the thread grooves. In this particular construction a series of springs is carried within the nut, selectively engageable with two of the balls, for holding each of the two in either of two oppositely-disposed extreme positions with respect to the nut. As the ends of the screw thread are reached by the balls, they are forcibly shifted by curved reversing grooves on the thread, so as to effect a reversal in the direction of movement of the nut. The above construction is much simpler and less costly, meeting with considerable commercial success. Its use, however, involves diamond-type screws which are more complex to machine and manufacture, resulting in a product which still has an appreciable overall cost that, while not so great as the cost of the roller-type transmission, was still prohibitive for many mass-market applications, as for example boat steering mechanisms, and other devices where small power is involved.
These prior devices all relied on a multiplicity of precision parts at the exact nucleus of the transmission, that is, where the engagement occurs between the relatively movable components which effect the change in motion. While precision balls are readily obtainable at fairly low cost, the related parts which engage and/or carry the balls involved precision forming, thereby obstructing efforts to effectively bring down costs to a commercially acceptable figure.